Kate Johnson

March 14, 2012

March 14, 2012 (Orlando, Florida) — Nearly half of asthmatic children had bronchiolitis during their first year, after their first exposure to respiratory syncytial virus (RSV) season, suggesting this cause of bronchiolitis might be a promising target for asthma prevention, according to a study presented here at the American Academy of Allergy, Asthma and Immunology 2012 Annual Meeting.

However, the attributable risk analysis of almost 260,000 children is based on the assumption that RSV bronchiolitis causes asthma, said Kristina James, MD, a fellow in allergy and immunology at Vanderbilt University School of Medicine in Nashville, Tennessee.

Roughly 70% of infants have been infected with RSV by the time they are 1 year of age, and 100% have been exposed by 2 years of age, she explained, yet only some of those viral infections progress to bronchiolitis.

Dr. James and colleagues — who previously showed a dose–response relation between the severity of infant bronchiolitis and the likelihood of developing asthma, and that the highest prevalence of asthma is in children who required hospitalization for infant bronchiolitis — set out to determine the attributable risk and population-attributable risk of developing asthma based on a history of bronchiolitis during the RSV season.

"We did not confirm the cause of bronchiolitis beyond the fact that it was during RSV season, so we could have missed other causes," she acknowledged.

The population-based birth cohort study involved 258,628 children born from 1996 to 2008; 70,648 were from the California Kaiser Permanente Medical Care Program (KPMCP) and 187,980 were from Tennessee Medicaid (TennCare).

The minimum gestational age of the eligible infants, who had no chronic lung disease, was 32 weeks.

A total of 32,550 children (13%) from age 4.5 to 6.0 years received a diagnosis of early asthma, which was defined using an algorithm of International Classification of Diseases, Ninth Revision (ICD-9) codes and asthma medication use, said Dr. James.

The researchers compared children with and without asthma. They found that almost double the number of children with asthma had had an infant bronchiolitis infection during RSV season, Dr. James reported.

Among the asthma patients, 16% of the KPMCP group and 23% of the TennCare group had a history of bronchiolitis; among nonasthmatic patients, the rates were 8% and 12%, respectively.

On the basis of these numbers, the researchers calculated the attributable risk of asthma from bronchiolitis during RSV season to be 50% in the KPMCP group and 46% in the TennCare group.

The population-attributable risk — the proportion of asthma in the total population that is attributed to bronchiolitis exposure — was calculated to be 10% in the KPMCP group and 12% in the TennCare group.

Despite significant differences in terms of race, maternal smoking, and maternal education, "what is very notable is how similar both the attributable risk and the population-attributable risk findings are in these 2 populations," said Dr. James.

The findings remained even after adjustment for possible covariates, including sex, race, gestational age, birthweight, maternal age, and maternal smoking.

Thus, preventing infant bronchiolitis during RSV season might be an effective asthma-prevention strategy, Dr. James explained.

In addition to the lack of laboratory confirmation of RSV as the cause of bronchiolitis, a limitation of the analysis is that it was confined to hospital records of bronchiolitis, which necessarily excluded mild cases that did not require treatment.

After hearing the presentation David Elkayam, MD, from the Bellingham Asthma, Allergy & Immunology Clinic in Washington, said these data "continue to strengthen the statistical association between RSV and asthma in children, but do not necessarily answer the question of whether RSV treatment or prevention prevents asthma. "We're still struggling in that regard," he told Medscape Medical News.

"There's a definite statistical correlation. Whether or not we can affect that with either antiviral or antibacterial medications remains to be seen," he said, adding that there is currently no preventive treatment for RSV.

"RSV is an equal-opportunity virus and will get all kids, all genders, all races," Dr. Elkayam explained. The underlying reason that some children go on to develop severe bronchiolitis and others do not may be the same reason that only some develop asthma.

"It's the chicken and egg question. Is this really their first asthma attack with the RSV, or does the RSV actually induce changes that lead to asthma? We're still unraveling that mystery."

Dr. James and Dr. Elkayam have disclosed no relevant financial relationships.

American Academy of Allergy, Asthma and Immunology (AAAAI) 2012 Annual Meeting: Abstract 922. Presented March 6, 2012.

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